The present invention relates to methods for making polyurethanes as thin films. As used herein, the term xe2x80x9cpolyurethane(s)xe2x80x9d includes both non-acrylated and acrylated polyurethanes as well as any other polyurethane-like polymers. In the art, classical polyurethanes are non-acrylated, and acrylated polyurethanes as well as other modified polyurethanes are considered polyurethane-like materials. For brevity, the term polyurethane(s) encompasses them all.
Polyurethanes are commonly used for coatings, adhesives, elastomeric materials, foams, and molding. Many of the mechanical properties of polyurethanes, like flexibility and adhesiveness, make them useful as thin films. As discussed by J. C. Salamone, editor, xe2x80x9cPolymeric Materials Encyclopediaxe2x80x9d, CRC Press, 1996, volume 9, pages 6940-6991, polyurethane materials are typically fabricated at atmospheric pressures via wet chemical processing methods such as reacting bischloroformate with a diamime or by reacting a glycol or alcohol with an isocyanate compound (typically a diol or dialcohol with a diisocyante). Disadvantages of these methods include inapplicability to vacuum deposition of thin films because the reactants are in a solvent (generally aqueous) that would boil upon introduction to a vacuum causing difficulty with the vacuum system and causing undesirable texture f the film. In addition these methods exhibit limited precision for controlled thickness, and limited production throughput. Depending on the specific solvent, catalyst, temperature, isocyanate (or diisocyanate), and alcohol or diol the reaction may proceed spontaneously, and quickly, at room temperature or the reaction may proceed relatively slowly. In most cases there is some degree of spontaneous reaction when the two principle reactants are placed in contact.
Several processes for making thin polymer layers have been developed, especially for electronic devices requiring polymer composite layers for devices including but not limited to molecularly doped polymers (MDP), light emitting polymers (LEP), and light emitting electrochemical cells (LEC). Presently these devices are made by spin coating or physical vapor deposition (PVD). Physical vapor deposition may be either evaporation or sputtering. With spin coating, surface area coverage is limited and scaling up to large surface areas requires multiple parallel units rather than a larger single unit. Moreover, physical vapor deposition processes are susceptible to pin holes.
Methods of liquid monomer spreading include but are not limited to physical or mechanical liquid:monomer spreading apparati, for example roll coaters, gravure roll coaters, wire wound rods, doctor blades and slotted dies. Thin films made by these methods are subject to pin holes and experience difficulties in bonding to additional layers. In U.S. Pat. No. 5,260,095 (herein incorporated by reference) there is described methods for liquid monomer spreading done under a vacuum or in a vacuum chamber, having advantages of avoiding or reducing the need for photoinitiator and also obtaining a smoother finished surface. U.S. Pat. No. 5,395,644 also describes a method of liquid monomer spreading under a vacuum but for the specific purposes of making lithium polymer batteries as well as for making electrochromic devices.
Another method of making thin polymer films is the process of plasma enhanced chemical vapor deposition (PECVD) described in THIN FILM PROCESSES, J. L. Vossen, W. Kern, editors, Academic Press, 1978, Part IV, Chapter IVxe2x80x941 Plasma Deposition of Inorganic Compounds, Chapter IVxe2x80x942 Glow Discharge Polymerization, herein incorporated by reference. Briefly, a glow discharge plasma is generated on an electrode that may be smooth or have pointed projections. Traditionally, a gas inlet introduces high vapor pressure monomeric gases into the plasma region wherein radicals are formed so that upon subsequent collisions with the substrate, some of the radicals in the monomers chemically bond or cross link (cure) on the substrate. The high vapor pressure monomeric gases include gases of CH4, SiH4, C2H6, C2H2, or gases generated from high vapor pressure liquid, for example styrene (10 torr at 87.4xc2x0 F. (30.8xc2x0 C.)), hexan (100 torr at 60.4xc2x0 F. (15.8xc2x0 C.)), tetramethyldisiloxane (10 torr at 82.9xc2x0 F. (28.3xc2x0 C.) 1,3,-dichlorotetra-methyldisiloxane) and combinations thereof that may be evaporated with mild controlled heating. Because these high vapor pressure monomeric gases do not readily cryocondense at ambient or elevated temperatures, deposition rates are low (a few tenths of micrometer/min maximum) relying on radicals chemically bonding to the surface of interest instead of cryocondensation. Remission due to etching of the surface of interest by the plasma competes with cryocondensation. Lower vapor pressure species have not been used in PECVD because heating the higher molecular weight monomers to a temperature sufficient to vaporize them generally causes a reaction prior to vaporization, or metering of the gas becomes difficult to control, either of which is inoperative.
The basic process of flash evaporation is described in U.S. Pat. No. 4,954,371 herein incorporated by reference. This basic process may also be referred to as polymer multi-layer (PML) flash evaporation. Briefly, a radiation polymerizable and/or cross linkable material is supplied at a temperature below a decomposition temperature and polymerization temperature of the material. The material is atomized to droplets having a droplet size ranging from about 1 to about 50 microns. An ultrasonic atomizer is generally used. The droplets are then flash vaporized, under vacuum, by contact with a heated surface above the boiling point of the material, but below the temperature which would cause pyrolysis. The vapor is cryocondensed on a substrate then radiation polymerized or cross linked as a polymer layer. The material may include a base monomer or mixture thereof, cross-linking agents and/or initiating agents. A disadvantage of the flash evaporation is that it requires two sequential steps, cryocondensation followed by curing or cross linking, that are both spatially and termporally separate.
According to the state of the art of making plasma polymerized films, PECVD and flash evaporation or glow discharge plasma deposition and flash evaporation have not been used in combination. However, plasma treatment of a substrate using glow discharge plasma generator with inorganic compounds has been used in combination with flash evaporation under a low pressure (vacuum) atmosphere as reported in J. D. Affinito, M. E. Gross, C. A. Coronado, and P. M. Martin, AVacuum Deposition Of Polymer Electrolytes On Flexible Substrates. xe2x80x9cPaper for Plenary talk in AProceedings of the Ninth International Conference on Vacuum Web Coatingxe2x80x9d, November 1995 ed R. Bakish, Bakish Press 1995, pg 20-36. In that system, the plasma generator is used to etch the surface of a moving substrate in preparation to receive the monomeric gaseous output from the flash evaporation that cryocondenses on the etched surface and is then passed by a first curing station (not shown), for example electron beam or ultra-Violet radiation, to initiate cross linking and curing. The plasma generator has a housing with a gas inlet. The gas may be oxygen, nitrogen, water or an inert gas, for example argon, or combinations thereof. Internally, an electrode that is smooth or having one or more pointed projections produces a glow discharge and makes a plasma with the gas which etches the surface. The flash evaporator has a housing, with a monomer inlet and an atomizing nozzle, for example an ultrasonic atomizer. Flow through the nozzle is atomized into particles or droplets which strike the heated surface whereupon the particles or droplets are flash evaporated into a gas that flows past a series of baffles (optional) to an outlet and cryocondenses on the surface. Although other gas flow distribution arrangements have been used, it has been found that the baffles provide adequate gas flow distribution or uniformity while permitting ease of scaling up to large surfaces. A curing station may be located downstream of the flash evaporator.
All of these deposition methods depend upon use of a monomer mixture (acrylated or non-acrylated) that is not reactive, or does not begin polymerizing prior to deposition.
In spite of these advances, there remains a need for methods that will enable more precise control of thickness of polyurethane films specifically, with increased production throughput.
The present invention is a method of making a thin film of a polyurethane. The method has the steps of:
(a) vacuum coating a thin layer of a solventless mixture of a monomer mixture, said monomer mixture selected from the group consisting of polyols, polyisocyanates and combinations thereof, onto a substrate in a vacuum chamber; and
(b) curing the monomer as said thin film of said polyurethane.
It is an object of the present invention to provide a method for making thin film of a polyurethane via vacuum coating.
In some instances it may be possible to simply mix the two reactants of polyol and polyisocyanate and run them through the vacuum deposition process exactly as would be done for a non-reactive acrylate monomer as described above. However, in most instances the spontaneous reaction (to form the urethane) would cause polymerization in a degas vessel or the monomer delivery plumbing and quickly clog the delivery system. When spontaneous reaction is a problem, each reactant may be delivered by separate degas/pumping systems to separate ultrasonic atomizers housed in the same vacuum chamber. Because the reactants spontaneously react at room temperature, the higher temperature of the flash evaporation process permits reduction or elimination of the use of catalysts.
It is to be understood that xe2x80x9csolventlessxe2x80x9d means either no solvent or an amount of solvent that is ineffective in altering the reaction.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.